Re: [PATCH 07/34] x86/entry/32: Restore segments before int registers

[Linus Torvalds]

On Mon, Mar 5, 2018 at 1:35 PM, Joerg Roedel <joro@xxxxxxxxxx> wrote:
> On Mon, Mar 05, 2018 at 12:50:33PM -0800, Linus Torvalds wrote:
>>
>> Ahh, good. So presumably Joerg actually did check it, just didn't even notice ;)
>
> Yeah, sort of. I ran the test, but it didn't catch the failure case in
> previous versions which was return to user with kernel-cr3 :)

Ahh. Yes, that's bad. The NX protection to guarantee that you don't
return to user mode was really good on x86-64.

So some other case could slip through, because user code can happily
run with the kernel page tables.

> I could probably add some debug instrumentation to check for that in my
> future testing, as there is no NX protection in the user address-range
> for the kernel-cr3.

Does not NX work with PAE?

Oh, it looks like the NX bit is marked as "RSVD (must be 0)" in the
PDPDT. Oh well.

                  Linux